They mean that you can't combine two options
That has Internet. FLEXI without Internet you can combine
With data only, no problems.

I had following experience with lebara:
1) I bought it in advance in Internet. Activated SIM card with 1€
Seller sent me username and password for online manager with phone number
2) than I login and tried to Topup from there, didn't succeed
3) I bought 20€ Topup code on guthaben.de
4) they email it to me after 1 minute
5) I called customer service 00 49 180 6 532272 (they spoke English) and I asked
To activate FLEXI 100 plus 3GB (together 20€) thank I gave him voucher code
And after 5 minutes all was set (I login and saw Internet and 100 minutes
And I get also free lebara to lebara calls and SMS

All worked very good (it is Tmobile network)
The downside was: call to customer service called to me about 1€ and not few cents as regular call to Germany.

in English, this UK-company is not able to translate: "The packages can only be booked optionally on a Lebara SIM. This option can only be booked once within 30 days. This tariff option can not be combined with other tariff options".